1. Field of the Invention
This invention relates to the use of Photonic Crystals as a source of electromagnetic radiation at THz or other frequencies, and more specifically to dynamically propagating a band gap discontinuity through a Photonic Crystal to modify the spectral distribution of thermal electromagnetic radiation.
2. Description of the Related Art
At quite modest temperatures, small objects possess significant free energy, which is emitted in the form of thermal electromagnetic radiation across a broad spectral band, including visible, infrared (IR), THz, etc. For a perfect blackbody emitter, the thermal electromagnetic radiation follows the Planck spectral distribution 10 shown in FIG. 1. The blackbody emitter has a primary emission wavelength (peak) 12 in the IR band around 5-10 μm, depending upon temperature. Common materials can have more complex emission spectra, but always maintain the Planck distribution as an upper limit to emission peaks rather than being distributed evenly across all frequency bands.
A Photonic Crystal (PC) structure contains a periodic high-contrast modulation of the local index of refraction (or dielectric constant, for non-magnetic materials) in one, two or three dimensions. The underlying materials, processing, fabrication and tuning mechanisms are well developed (see for example C. Lopéz, Advanced Materials 15, 1679 (2003)). Any two substances having sufficient contrast between their respective indices of refraction can be placed in a stable periodic arrangement with a particular geometry, spacing and shapes of the constituent substances to create a photonic band gap (PBG) for a particular range of photon wavelengths. Electromagnetic radiation propagating in such a structure will undergo multiple Bragg scattering from the lattice array. Under certain conditions, the multiply-scattered waves interfere destructively, resulting in minimal transmission over a broad range of wavelengths, which is termed the “band gap” (a term borrowed from semiconductor physics). The PBG is said to be complete when transmission is blocked for all polarizations and all angles of incidence within the wavelength band. The PC material can be actively controlled to open or close the PBG, or shift the edges of the band gap. This can be accomplished by modulating the index of refraction contrast, changing the geometric arrangement or altering the symmetry of the scattering objects. If the periodic lattice is strained in such a manner as to maintain its periodicity but change its lattice spacing, the deformation simply alters the symmetry parameters thereby shifting the edges of the band gap. Or, if the periodic lattice is strained in such a manner to create aperiodic lattice deformation, the band gap can be switched off.
An object's Planck spectral distribution may be modified when the object is a PC. The existence of a PBG can be used to suppress radiated power in certain wavebands and enhance radiated power in other wavebands (Z. Li, Physical Review B 66, R241103 (2002), and S. Lin et al., Physical Review B 62, R2243 (2000)). This effect is most obvious when the band gap is positioned around the main peak (e.g., 5-10 μm) of the Planck spectral distribution. A three-dimensional (3D) PC can induce strong redistribution of the photon Density of States (DOS) among different frequency bands to modify the thermal electromagnetic radiation. Li supra designed the DOS of a PC to provide orders-of-magnitude enhancement in the low-DOS band of the short-wavelength region. This leads to significantly enhanced emission of thermal electromagnetic radiation in the visible waveband (approximately 0.5 μm) for a modest cavity temperature.
E. J. Reed et al., Physical Review Letters 90, 203904 (2003a); E. J. Reed et al., Physical Review Letters 91, 133901 (2003b); and E. J. Reed et al. in U.S. Pat. No. 6,809,856 entitled “Shock-wave Modulation and Control of Electromagnetic Radiation” and issued on Oct. 26, 2004 describe a technique for frequency shifting (upward or downward) single-frequency electromagnetic radiation injected into the PC from an external single-frequency laser. Reed et al. consider a particular form of a temporally-varying PC by simulating the propagation of a lattice-distorting (or index-of-refraction-changing) pulse (“shockwave”) in a PC having a PBG. The incident light encounters the shock wave (i.e., the upshifted first band gap) moving in the opposite direction and is reflected backwards so that the light rides up on the shock wave and is pushed over the band gap to a higher frequency. In their implementation, the incident radiation was confined to the visual spectrum with frequency shifts limited to the width of the PC's first static band gap. In a later publication (Reed 2003b) they illustrate shifting frequencies downward by a compression induced wave in the dielectric constant of the material.
One of the major bottlenecks for the successful implementation of THz-frequency systems has been the limited output power of conventional THz sources. Most systems produce THz radiation via optical techniques, but those require massive lasers, complex optical networks and cooling systems. There are other reports on generating THz radiation using PC structures. Lu et al., IEEE Journal of Quantum Electronics 38, 481 (2002), discuss optical rectification in a nonlinear PC, requiring pump light and converting it to THz with an efficiency of approximately 1%. Iida et al., CLEO-QELS 2003 Conference Paper CMI2 (2003), place a photomixing antenna in a high-Q defect cavity in a PC.
None of these approaches offers a compact, reliable and low-cost radiation source, nor the bandwidth and frequency agility to source electromagnetic energy at THz and other frequencies.